Foliar optical traits capture physiological and phenological leaf plasticity in Tilia×euchlora in the urban environment

Knowledge on the response of trees to the urban heat island (UHI) effect and soil sealing is currently limited, yet of vital importance in an era characterized by both climate change and urbanization. We investigated the physiological and phenological leaf plasticity of Tilia×euchlora trees to the UHI effect and soil sealing and explored the potential of leaf optical traits to quantify the magnitude of leaf plasticity. Temporal changes of leaf water content (LWC), specific leaf area (SLA), total chlorophyll (Chl) and carotenoids (Car) content, Car:Chl ratio and leaf reflectance for 46 Tilia×euchlora trees were measured along a soil sealing and urbanization gradient. The leaf functional traits displayed trait-specific temporal patterns during the growing season. We observed higher LWC and SLA but lower Chl and Car contents in the coolest zones. We found earlier autumn downregulation in Chl and Car content at paved sites compared to unsealed sites (maximum difference = 13 days). The magnitude of plasticity in relation to the UHI and soil sealing varied in leaf functional traits with largest variation observed in Chl (38%), followed by Car:Chl (31%), Car (29%), SLA (26%) and LWC (8%). The proposed spectral indices calculated using leaf reflectance measurements were able to track the spatiotemporal variations and phenology in the leaf functional traits. Our results clearly demonstrate the leaf plasticity of Tilia×euchlora trees, which provides Tilia×euchlora trees the necessary capacity to adapt to rapid changes in the urban environment. More importantly, we demonstrated the suitability of leaf optical traits to serve as a proxy of leaf functional traits for studying the spatiotemporal response of urban trees to environmental factors, which opens up new possibilities for large scale ecological studies using remote sensing.